The packaging of micro devices is a complex field of endeavor. As used herein, the term “micro device” refers collectively to (1) microelectronic devices, such as semiconductors and integrated circuits, (2) micromechanical devices, which include static micro-machined structures such as micromirrors, multiplexers, cross-connects, optical filters and attenuators, and (3) microelectro-mechanical systems (MEMS), which include moving micro-machined structures, such as configurable micromirrors, switches, configurable multiplexers, optical filters and attenuators, and the integrated circuits used to control them.
Typically, a micro device is packaged in a housing which has a cavity to contain the micro device and an aperture through which the micro device is inserted into the cavity. The aperture of the housing is capped by a lid or cover. In the case of optical micro devices, the lid may comprise an optically clear material such as quartz or glass.
Mating and fastening the lid to housing presents a number of challenges. Among other considerations, the seal between the housing and lid must be capable of withstanding the mechanical stresses induced by vibration and by differences in the thermal expansion characteristics of the various materials used in the package. Often such stresses are sufficient to break the seal, or otherwise stress the cap to the point it cracks or develops unwanted conditions. Furthermore, in certain applications, a micro device package must be hermetically sealed. For example, it may be necessary to insulate the micro device from humidity and external elements which can degrade its performance or life, or to contain a special atmosphere within the housing to improve the performance of the micro device. A hermetic package requires that the seal between the housing and lid be not only robust, but also impervious to gaseous contaminants that could impede the operation of the micro device if they leaked into the cavity. Hermetically-sealed micro device packages are of particular interest herein.
To form a hermetic seal between the housing and the lid, it is common practice to use a metallic seal—i.e., a solder seal—between the housing and the lid. The solder seal is achieved customarily by treating the ceramic to form a narrow layer of tungsten. Tungsten is a well-known surface preparation for ceramic materials as it provides a good interface between the ceramic material and other metals, such as nickel, which are used in forming a solder seal. In this respect, nickel is also well known as a “primer” or base metal upon which other materials can be layered to facilitate a solder seal.
Although nickel is an excellent interface between tungsten and other metals for a solder seal, it tends to be magnetic which can be problematic. For example, in magnetic resonance imaging (MRI), any magnetic properties of the electronics used in the imaging equipment can negatively affect the quality of the images produced. Consequently, metallurgies using iron (Fe), nickel (Ni) or cobalt (Co) and their alloys cannot be used in micro device packaging if completely non-magnetic properties are to be achieved. Recently, this need has been addressed by attempting to render Ni non-magnetic by alloying it with other materials such as copper. To this end, current technologies employ nickel and copper deposited electrochemically in multiple thin layers. The multiple layers are thermally sintered to form a quasi non-metallic alloy. Although, theoretically, the alloy formed should be non-magnetic, applicants have discovered that this is not the case, and that micro device packages using this approach still have detrimental magnetic properties.
Therefore, there is a need for a micro device package which is not only hermetically sealed, but also non-magnetic. The present invention fulfills this need among others.